The present invention relates to a combined semiconductor rectifying device. Specifically, the present invention relates to a combined semiconductor rectifying device that includes a wide-band-gap semiconductor rectifying device (hereinafter referred to as a “WBG semiconductor rectifying device”) including a drift region made of a wide-band-gap semiconductor (hereinafter referred to as a “WBG semiconductor”), the band gap thereof is wider than the band gap of silicon, and a silicon semiconductor rectifying device connected in series to the WBG semiconductor rectifying device. The invention relates also to an electric power converter that employs the combined semiconductor rectifying device described above.
In inverter circuits and switching power supply circuits, a switching device is turned on and off at a high frequency. In those circuits, a semiconductor rectifying device (diode) connected in opposite parallel to the switching device and working for a freewheeling diode (hereinafter referred to as an “FWD”) also conducts a high-speed switching operation. It is required especially for the semiconductor rectifying device that conducts the high-speed switching operation to exhibit a breakdown voltage as high as the breakdown voltage of the switching device and a reverse recovery time (designated as “trr”) as short as the reverse recovery time of the switching device. It is also required for the semiconductor rectifying device to cause a low reverse leakage current IR and a low forward voltage VF.
The reverse recovery time trr of an ordinary PN-junction silicon diode of the 600 V breakdown voltage class is relatively long at the ordinary temperature and long at high temperatures. Due to the long reverse recovery time trr, it is not advantageous to use the PN-junction silicon diode for the FWD described above. However, the PN-junction silicon diode is advantageous due to the reverse leakage current IR thereof smaller than the reverse leakage current IR caused in the wide-bang-gap Schottky barrier diode (hereinafter refereed to as a “WBG-SBD”).
In the field of power semiconductor, WBG diodes manufactured using a WBG semiconductor substrate are increasing. Although any product of a high-breakdown-voltage silicon SBD of the 600 V class has not been manufacture yet, the products of WBG-SBDs such as a gallium nitride (GaN) SBD and a silicon carbide (SiC) SBD have been manufactured already. Since these SBDs exhibiting a high breakdown voltage are the devices of a unipolar carrier type, any reverse recovery time trr due to the accumulation of minority carriers is not caused theoretically. Therefore, it is a specific feature of the high-breakdown-voltage SBDs that any variation is not caused in the reverse recovery time trr at high temperatures. However, the reverse leakage current IR of the high-breakdown-voltage SBD higher than the reverse leakage current IR of the PN-junction silicon diode of the same breakdown voltage class is troublesome sometimes.
The circuit configurations that include semiconductor devices, including any of the WBG semiconductor devices described above and connected in series to each other, and the performance improvements of the semiconductor rectifying device based on the series connection structure thereof are described in the following patent documents. Japanese Unexamined Patent Application Publication No. 2005-276979 describes an LED light source having a configuration that connects a GaN LED and Si diodes, connected in series for raising the breakdown voltage, in series in the forward direction. Japanese Unexamined Patent Application Publication No. 2008-198735 describes a combined semiconductor device working for a diode that includes a silicon diode and a unipolar-type control semiconductor device connected in series to each other for improving the reverse recovery time, the breakdown voltage, and the ON-state resistance. Japanese Unexamined Patent Application Publication No. 2004-214268 describes the series connection of a silicon SBD and an SiC diode that facilitates manufacturing a combined diode exhibiting a high breakdown voltage.
Although the combined semiconductor devices described in the above documents employ a series connection of semiconductor devices including a compound semiconductor device, the problems which the combined semiconductor devices intend to solve are different from each other. Any description that suggests the use of the combined semiconductor devices in an inverter circuit or in a switching power supply circuit is not found in the documents. The combined semiconductor device described in the Japanese Unexamined Patent Application Publication No. 2008-198735 is used for a freewheeling diode, and discloses a configuration that makes a series connection of a silicon semiconductor device, exhibiting a low forward voltage drop and a low breakdown voltage, and a control semiconductor device, exhibiting a high breakdown voltage and working equivalently for a resistor in the ON-state thereof, work for a diode. The targets of the combined semiconductor device described in Japanese Unexamined Patent Application Publication No. 2008-198735 are improvements of the forward voltage drop, the breakdown voltage and the reverse recovery time. However, some problems have been left unsolved so far in manufacturing the control semiconductor device using a compound semiconductor economically with low manufacturing costs.
In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to provide a high-breakdown-voltage combined semiconductor rectifying device that exhibits a shortened reverse recovery time and low reverse leakage current characteristics. It would be further desirable to provide an electric power converter that employs the combined semiconductor rectifying device that exhibits the improved performances as described above.